The unique design of a Wankel rotary engine creates some special problems with respect to heat dissipation. Conventionally, excessive heat absorbed by the materials from the rotary engine is extracted by both a water cooling system as well as the oil lubrication system, each having their own cooler or heat exchanger. The use of oil as a mechanism for extracting heat is considerably different from the use of oil in a piston engine.
To lubricate the Wankel rotary engine, one part of the oil is injected in the air-fuel mixture to lubricate the wall of the epitrochoid surface and the seals and another part of the oil is circulated onto the shaft bearings and shot into the rotor to cool same. The most important quality which the oil should possess is the ability to contribute to the heat balance of the engine. Typically the oil used in a piston engine will carry only 12% of the heat loss that is extracted by the water cooling system. In contradistinction the oil lubricating system of a Wankel engine will carry away at least 50% of the heat removed from the engine by the water cooling system and will dissipate 7% of the total energy used by the engine. Important oil temperature variations are thus observed in service within a rotary engine. At full power, the oil in the sump can easily reach temperatures of about 150.degree.C., not withstanding external ventilation. The oil inside the rotor certainly must reach even higher temperatures.
Another problem associated with the oil lubrication system of a Wankel type engine is that of the influence of lubricating oil on the possibility of pre-ignition. The heat of the combustion process is conventionally conducted from the walls of the rotary piston so that the exposed surface of the rotor may act as a chill element to the combustible gases in the variable volume chamber. As a result, a thin layer of such gases against the rotor remains uncombusted to contribute to lower efficiency, fuel consumption and poor emissions. If the rotor were allowed to build up heat on the exposed surface, local hot spots would result in pre-ignition or "popping" of the air-fuel mixture prior to suitable ignition timing. This can be very detrimental to the serviceability of the engine. Since the variable combustion chambers of a rotary engine pass over all points of the rotor housing, only a single source of ignition is required for each rotor. Any misfiring causing by hot spots or deposits may disturb the cycle harmony and thus is one of the traditional handicaps of a rotary engine.
A related problem to unnecessary heat extraction, is the emissivity level of a typical Wankel engine. The hotter the temperature of the exhaust gases, the more effective will be control of the carbon monoxide and hydrocarbons. However, due to the excessive heat loss through the oil lubricating system, the temperature of combustion is lowered and the emissions not lowered.